To examine knocking during combustion in an Otto engine optical sensors are employed for example. They measure the brightness within the combustion chamber and thus provide a clue with respect to the time sequence of the events during combustion. For this purpose, optical sensors are for example arranged in spark plugs. However, these have a tendency to accumulate dirt after a short time for which reason optical sensors are sometimes mounted in the central electrode of sensors. During ignition the surfaces are subject to high temperatures whereby they are cleaned because the soot accumulated thereon is burned by the heat.
Spark plugs of the type mentioned in the beginning are known. DE 3001711 by Robert Bosch GmbH details various embodiments wherein for example a sapphire rod is used or a bundle of optical fibers. A disadvantage of these arrangements is that the sapphire rod has a very narrow opening angle and that in addition the optical fiber bundle cannot be assigned to the exact positions.
It has to be pointed out that for the examining of knocking a large surface area around the spark plug must be analyzed.